The functional and chemical properties of vegetable proteins are complex. Vegetable seed materials contain a diverse mixture of protein molecules and protein aggregates. A typical commercial vegetable seed protein extract contains a mixture of 2S, 7S, 11S and 15S proteins which respectively correspond to peak molecular weights of approximately 25,000, 160,000, 350,000 and 600,000, as analyzed by ultracentrifugation. In the commercial manufacture of isolates, these protein components are typically isolated from the vegetable seed water-solubles by a precipitating pH adjustment within the 4.2-4.6 range. Other vegetable seed protein components are the whey proteins. Due to the processing conditions conventionally used in the isolate manufacture, the whey proteins are normally not present in the commercial isolate product. The whey proteins are water-soluble molecules of a low molecular weight which remain in aqueous solution within the pH 4.2-4.6. A fraction identified as 9S is believed to be a dimer of the 7S fraction formed under conditions of low ionic strength. A typical commercial extraction process will yield a protein mixture comprised of approximately 22% 2S, 37% 7S, 31% 11S and 11% 15S.
Complex laboratory techniques have been devised for isolating and characterizing these diverse protein fractions. Such techniques, however, are impractical for a commercial operation. It has been observed that minor differences in the fractionation process often significantly alter the character and composition of the isolated product.
Smith et al. (Peptization of Soybean Proteins--Extraction of Nitrogenous Constituents from Oil-Free Meal by Acids and Bases with and without Added Salts--Industrial & Engineering Chemistry, Vol 30, No. 12, 1938, pages 1414-1418) disclose a solubility curve for the extraction of soy proteins under varying pH conditions. The minimum soy protein extraction was reported to be within the pH 4.0-5.0 range.
U.S. Pat. No. 4,188,399 by Shemer discloses extracting an enriched 7S protein fraction from defatted soybean materials by conducting the extraction within the pH 5.1-5.9 range. The enriched 7S isolate fraction is recovered by adjusting the extract to pH 4.5 which precipitates the 7S along with extracted 2S and 9.9S proteins. British Patent Specification No. 1,377,392 reports the precipitation of protein isolates from the aqueous defatted soybean extracts in the presence of water-soluble sulfite, bisulfite or dithionite salts. U.S. Pat. No. 2,489,208 (J. R. Turner) discloses the extraction of glycinin at a pH 6.4-6.8 which is then precipitated from the extract by a pH adjustment with sulfur dioxide to a pH 4.2-4.6 range.
Thanh et al. (Jr. Agr. Food Chem., Vol. 24, No. 6, 1976, pages 1117-1121) discloses a preparative technique for precipitating 11S from a crude extract of 7S and 11S soy globulin mixture by adjusting the protein extract buffered with dilute tris(hydroxymethyl)aminoethane to a pH 6.6. The acid-precipitated 11S protein is recovered by centrifugation and the 7S protein plus other protein extracts are isolated from the supernatant by pH 4.8 acid-precipitation. The 7S precipitate was redissolved in water, buffered with tris and adjusted to pH 6.2 to solubilize the 7S therefrom. The pH, ionic strength, tris buffer and protein concentration reportedly affect the efficacy of the 7S and 11S fractionation.
Other references reporting upon the 7S, 11S separation include: "Purification of the 11S Components of Soybean Protein" by Eldridge et al. (Cereal Chemistry, Vol. 44, November 1967, pages 645-652), "An Electrophoretic Analysis of Soybean Protein" by Briggs et al. (Cereal Chemistry, Vol. 27, May 1950, pages 243-257), "Purification and Characterization of the 11S Component of Soybean Proteins" by Wolf et al. (Archieves of Biochemistry and Biophysics 85, pages 186-199 (1959)), U.S. Pat. No. 3,953,611 by Youngquist and U.S. Pat. No. 3,870,801 by Tombs.
Numerous patents have also reported the enzymatic treatment of vegetable seed proteins for purposes of modifying the protein properties. Illustrative patents pertaining to the enzymatic modification of vegetable seed proteins include U.S. Pat. Nos. 1,373,651 by Cullen, 2,381,407 by Levinson et al., 3,814,816 by Gunther, 2,502,029 and 2,502,482 by Sair et al. Heretofore enzymatic modifications have typically been conducted upon protein mixtures. The Gunther patent discloses enzymatically modified soy proteins useful as a vegetable aerating protein and as a casein or egg replacement for a variety of applications. Pursuant to the Gunther patent, vegetable proteins are extracted and hydrolyzed (acid or alkali) and subsequently treated with pepsin to provide an enzymatically modified vegetable protein hydrolyzate. Except for the Gunther patent, the commercial success of the enzymatically modified proteins has been limited.